Lock Front/Top/Left views from entering User view when dragging mouse
Well. it turns out there is an add-on for this already that lets you lock/unlock any 3D view (top/bottom, left/right, front/back views.. and also the user view.. in orthographic or perspective modes). The add-on is available on Blender Market:
https://blendermarket.com/products/wlock-pro---an-addon-which-allows-to-lock-3d-view-rotation